ABSTRACT Hemolytic diseases, including sickle cell disease (SCD) and ?-thalassemia (THAL), are generally hallmarked by hemolysis and chronic inflammation. The mechanism underlying the different extent of inflammation in these diseases is unknown (SCD>THAL). Our published findings revealed that, besides promoting vascular endothelium activation, free heme triggers a M1-like pro-inflammatory phenotypic switching of reticulo- endothelial macrophages which alters their functional properties. Recently, we observed that the exposure to intact red blood cells (RBCs), and therefore erythrophagocytosis, induces a M2-like anti-inflammatory macrophage phenotype. This led us to hypothesize that the differential degree of inflammation between SCD and THAL is determined by the type of hemolysis, either intra-vascular (SCD) or extra-vascular (THAL), with different effects on macrophage functions. In Aim 1 we will address the concept that whereas in SCD inflammation is triggered by heme-induced M1-like macrophages, in THAL it is in part ?blunted? by erythrophagocytosis-induced M2-like macrophages. We believe that this mechanism similarly underlies the anti-inflammatory effect of fresh RBC transfusions and the detrimental effect of old RBC transfusions in SCD through the exposure of macrophages to heme in intact RBCs or in free form, respectively. To test this hypothesis we will take advantage of mouse models of intra- and extra-vascular hemolysis and compare SCD and THAL mice, as well as analyze another hemolytic condition, Babesia infection, and study the differential phenotypic switching of tissue macrophages induced by these events on macrophage activation and inflammation. Finally, we will interrogate cell metabolome, with the hypothesis that free heme promotes M1 polarization via TRAF6/NF-kB and/or Akt/mTORC1/HIF1?-triggered glycolytic switching. We further posit that this mechanism has relevance for acute chest syndrome (ACS), a main cause of morbidity and mortality in SCD. In Aim 2 we will address the central hypothesis that by mediating lung macrophage inflammatory phenotypic switching, heme promotes ACS and exacerbates symptoms in response to infections and fat emboli. To address this, we plan to assess the contribution of heme-induced M1 macrophages to ACS pathophysiology in SCD mice and validate the findings in SCD patients under steady-state and ACS condition by analyzing sputum macrophage polarization/function with plasma/sputum heme and cytokines levels and studying the functional alterations induced by free heme exposure in sputum macrophages of steady-state SCD patients. Finally, we will test whether transfusions, metabolic modulators and adoptive transfer of macrophages with increased iron export ability and anti-inflammatory/anti-hemolytic therapies can mitigate ACS, by triggering or reprogramming macrophages towards an anti-inflammatory phenotype. Our translational study is expected to (i) shed light on the mode of action of heme as lung stressor in ACS, with a focus on the newly observed heme-activated macrophages; (ii) improve transfusion practice in SCD and THAL and understand its beneficial effect in ACS; (iii) identify new targets and test strategies to implement the current therapeutic options. Our goal is to propose novel targeted therapies for chronic inflammation and ACS based on the modulation of macrophage polarization.